The present invention relates to a hydrocarbon conversion process. More particularly, this invention relates to the catalytic hydrocracking of hydrocarbons.
The hydrocracking of hydrocarbons is old and well known in the prior art. These hydrocracking processes can be used to hydrocrack various hydrocarbon fractions such as reduced crudes, gas oils, heavy gas oils, topped crudes, shale oil, coal extract and tar extract wherein these fractions may or may not contain nitrogen compounds. Modern hydrocracking processes were developed primarily to process feeds having a high content of polycyclic aromatic compounds, which are relatively unreactive in catalytic cracking. The hydrocracking process is used to produce desirable products such as turbine fuel, diesel fuel, and middle distillate products such as naphtha and gasoline.
The hydrocracking process is generally carried out in any suitable reaction vessel under elevated temperatures and pressures in the presence of hydrogen and a hydrocracking catalyst so as to yield a product containing the desired distribution of hydrocarbon products.
Hydrocracking catalysts generally comprise a hydrogenation component on an acidic cracking support. More specifically, hydrocracking catalysts comprise a hydrogenation component selected from the group consisting of Group VIB metals and Group VIII metals of the Periodic Table of Elements, their oxides or sulfides. The prior art has also taught that these hydrocracking catalysts contain an acidic support comprising a large-pore crystalline aluminosilicate material such as X-type and Y-type zeolites. This large-pore crystalline aluminosilicate material is generally suspended in a refractory inorganic oxide such as silica, alumina, or silica-alumina. The preferred Group VIB metals are tungsten and molybdenum while the preferred Group VIII metals are nickel and cobalt.
The prior art has also taught that combinations of metals for the hydrogenation component, expressed as oxides and in the order of hydrogenation activity, are: NiO-WO.sub.3, NiO-MoO.sub.3, CoO-MoO.sub.3, and CoO-WO.sub.3.
Other hydrogenation components broadly taught by the prior art include iron, ruthenium, rhodium, palladium, osmium, indium, platinum, chromium, molybdenum, vanadium, niobium, and tantalum.
With reference to the acidic support the prior art is replete with disclosures of zeolites suitable for incorporation into hydrocracking catalysts. For instance, U.S. Pat. No. 4,600,498 (Ward) discloses the use of faujasite, mordenite, erionite, zeolite Y, zeolite X, zeolite L, zeolite Omega, ZSM-4 and their modifications.
In accordance with the teachings of the U.S. Pat. No. '498, a preferred zeolite is zeolite Y and its modifications as disclosed in U.S. Pat. No. 3,130,007 (Breck). These include zeolite Y in the hydrogen form and zeolite Y that has been ion-exchanged with ammonium ions and then steam stabilized.
A preferred zeolite used in hydrocracking catalysts is the ultrastable zeolite type Y. These zeolites generally maintain their crystalline structure during high temperature and steam treatments. The subject zeolites are further characterized by an R.sub.2 O content of less than 1% by weight where R designates any alkali metal ion. These zeolites also possess a silica to alumina molar ratio that varies from about 3.5 to about 7 and a reduced unit cell size. The ultrastable zeolite type Y is typically prepared by carrying out successive base exchanges with a type-Y zeolite utilizing an aqueous solution of an ammonium salt such as ammonium nitrate until the desired low alkali metal content is achieved.
The exchanged zeolite Y is then washed and calcined in the presence of steam at about 540.degree. C. to about 815.degree. C. to produce an ultrastable zeolite Y. The above sequence of ion exchange followed by calcination can be repeated until the final zeolite possesses the desired alkali metal content and unit cell size.
A specific ultrastable type Y zeolite employed in hydrocracking catalysts is the ultrastable, large pore crystalline aluminosilicate material designated as Z-14 U.S. zeolite which is described in U.S. Pat. Nos. 3,293,192 (Maher et al.) and 3,449,070 (McDaniel et al.). These zeolites possess a low alkali metal content, are ammonia-free, and posses a silicon-rich framework demonstrated by a smaller unit cell size than the ammonia-exchanged zeolite Y. It should be noted that these zeolites generally possess the same amount of aluminum, however, there is less aluminum in the framework and more in the amorphous phase.
U.S. Pat. No. 4,816,538 (Abdo) further discloses the use of zeolitic molecular sieves such as ZSM-5, fluorided Y zeolites, and zeolite Beta in hydrocracking catalysts. The subject patent also generally discloses the use of nonzeolitic crystalline molecular sieves in hydrocracking catalysts such as silicoalumino-phosphates, alumino phosphates, ferrosilicates, titanium alumino silicates, borosilicates and chromosilicates.
The subject patent further discloses the use of a particular Y zeolite that has been dealuminated to achieve a silica to alumina molar ratio of about 6.0. A preferred dealuminated Y zeolite is "LZ-210" available from the Linde Division of the Union Carbide Corporation. The unit cell size is at or below 24.65 Angstroms and normally ranges between 24.20 and about 24.65 Angstroms. These zeolites have a capacity for water vapor at least 20 wt. % based on the anhydrous weight of the zeolite.
As can be appreciated from the above, there is a myriad of catalysts known for hydrocracking whose catalytic properties vary widely.
Broadly, it has now been discovered that if a precise combination of two different ultrastable type-Y zeolites are present in the hydrocracking catalyst in accordance with the present invention, both the catalyst activity and the selectivity towards naphtha can be markedly improved when carrying out a hydrocracking process. The improvements in activity or naphtha selectivity are not realized if either ultrastable zeolite Y is present alone in the hydrocracking catalyst, or if both zeolites are present in proportions not in accordance with the present invention.